Legendary physicist Stephen Hawking has died at age 76, according to his family. Hawking's work on theoretical physics earned him countless honors, and made him a key voice in understanding our universe.

Here are five things to know about Hawking's life and career.

1. He was diagnosed with ALS at 21

The disease forced Hawking to remain in a wheelchair and depend on a computerized voice system to communicate. According to Live Science, the average life expectancy after an ALS diagnosis is three years, with only 5% living 20 years or more. Genetics could have played a key role in Hawking's survival, Live Science reports. Studies suggest people diagnosed at a younger age tend to live much longer.

2. One of his biggest discoveries involved black holes

Working with fellow scientist Roger Penrose, he showed Albert Einstein's theory of relativity suggested space and time had a definitive beginning and end. This led to the theory that black holes aren't completely black, but emit radiation and eventually disappear.

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3. His book, 'A Brief History of Time,' was a record-breaker

In 1988, Hawking published a book targeting non-scientists to help explain fundamental questions such as how the universe started and whether it will end. It set a record by spending more than five years on The Sunday Times bestseller list, and was the No. 1 all-time book on their list as of 2014.

4. He held 12 honorary degrees

Among Hawking's other accolades: In 1982, he received the Commander of the Order of the British Empire honor and was awarded the Presidential Medal of Freedom in 2009. He was also named a Fellow of The Royal Society, comprised of "the most eminent scientists, engineers and technologists from the UK and the Commonwealth," according to the society.

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5. He was really worried about the future of humanity

Hawking expressed concern that the world's citizens could consume enough energy to turn Earth into a ball of fire within 600 years. That is, unless the rise of artificial intelligence doesn't get to us first.

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British Scientist Stephen Hawking, a celebrated theoretical physicist and cosmologist is the author of "A Brief History of Time" and his black hole thermal radiation discovery in 1974 is considered to be one of the most important physics results of the past century. Professor Hawking has a motor neuron disease that has left him completely paralyzed. Seen hereStephen Hawking delivers a keynote speech as he receives the Honorary Freedom of the City of London, March 6, 2017. Hawking was presented the City of London Corporation's highest award Monday in recognition of his outstanding contribution to theoretical physics and cosmology.
Matt Dunham, AP

Russian billionaire Yuri Milner, left, and British scientist Stephen Hawking, center, during a press conference in London on July 20, 2015. Milner and Hawking announced a global science initiative for the search of civilized life in the universe.
Andy Rain, EPA

Professor Stephen Hawking and Eddie Redmayne attend the U.K. premiere of "The Theory Of Everything" at Odeon Leicester Square on Dec. 9, 2014, in London. Redmayne won the 2015 Oscar for best performance by an actor in a leading role for his portrayal of Hawking in the biopic.
Karwai Tang, WireImage

A handout picture released by the London Science Museum on Jan. 5, 2012, and taken on Dec. 14, 2011, shows British scientist Stephen Hawking in his office at the University of Cambridge where he is director of research for the Department of Applied Mathematics and Theoretical Physics and founder of the Centre for Theoretical Cosmology. The Science Museum commissioned a series of photographic portraits of Hawking as part of an exhibition to honor the eminent professor as he turned 70 on Jan. 8, 2012.
AFP Photo/London Science Museum/Sarah Lee, Getty Images

President Obama presents the Medal of Freedom to physicist Stephen Hawking during a ceremony in the East Room of the White House Aug. 12, 2009, in Washington, D.C. Obama presented the medal, the highest civilian honor in the United States, to 16 recipients during the ceremony.
Chip Somodevilla, Getty Images

Dr. Stephen Hawking, professor of mathematics at the University of Cambridge, and his daughter Lucy Hawking stand on stage April 21, 2008, at George Washington University's Morton Auditorium in Washington, D.C. Hawking gave a speech entitled "Why we should go into space" as part of a lecture series marking NASA's 50th anniversary.
Paul E. Alers/NASA via Getty Images

Professor Stephen Hawking delivers his speech at the release of the "Bulletin of the Atomic Scientists" on Jan. 17, 2007, in London. A group of scientists assessing the dangers posed to civilization have moved the Doomsday Clock forward two minutes closer to midnight as an indication and warning of the threats of nuclear war and climate change.
Bruno Vincent, Getty Images

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Stephen Hawking was almost as good at comedy as he was at physics. After he made his first cameo on ‘Star Trek: The Next Generation,’ he became a popular guest star on ‘The Simpsons’ and ‘The Big Bang Theory’ before getting the biopic treatment.
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Stephen Hawking may have just made a huge breakthrough when it comes to black holes and it turns out, it's possible that there is an escape from the famed event horizon. Patrick Jones (@Patrick_E_Jones) has the details.
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